The present invention relates to a camera module for electronically recording images, and particularly to a module having an image sensor with a plurality of image cells, with each image cell providing an electric image signal as a function of the intensity of incident light, each image cell having a light-sensitive element for generating a light-dependent current and at least one MOS transistor arranged in series therewith, a gate of the transistor being at a fixed potential and a source-drain path of the transistor being flowed through by the light-dependent current.
The invention further relates to a method for electronically recording images by means of an image sensor having a plurality of image cells, the method comprising the following steps:                generating a light-dependent current by means of a light-sensitive element in each of the image cells,        generating an electric image signal in each of the image cells by means of an MOS transistor which is arranged in series with the light-sensitive element and whose gate is at a fixed potential and whose source-drain path is flowed through by the light-dependent current, and        reading out the electric image signals of all the image cells.        
Image sensors for electronic recording of images have already found commercial application for some years in so-called digital cameras. The image sensors have a plurality of image cells which are arranged relative to one another in an array like fashion and which are frequently called pixels. The individual image cells generate an analogue electric image signal as a function of the intensity of incident light, which analogue image signal is subsequently converted into a digital image signal by means of an analogue-to-digital converter. The digital image signal is subsequently available for further processing and/or for outputting on a monitor, a printer or the like.
EP 0 632 930 B1 discloses an image cell for an image sensor which is characterized, in particular, by its wide-ranging logarithmic dependence between the intensity of incident light and the magnitude of the analogue image signals generated. This known image sensor thereby permits contrasting recording of image scenes with extremely strong brightness differences. In other words, the known image sensor has a very wide dynamic range. This characteristic is mainly achieved by a particular connection of the light-sensitive element which is coupled to an MOS transistor such that the photocurrent generated by the light-sensitive element flows through the source-drain path of the MOS transistor. The gate of the MOS transistor is shorted to the drain electrode of the MOS transistor, and this leads to the advantageous logarithmic dependence.
However, the known image cell has disadvantages under specific operating conditions. In particular, the transient response of the image cell is relatively slow in the case of a quick change from a light signal of high intensity to a light signal of low intensity (i.e. from bright to dark). As a consequence, when a bright moving object is being recorded against a dark background, so-called pulling effects occur which in the case of a fast image sequence are visible in the form of a light effect similar to a comet's tail. In fact, however, there is actually no comet's tail present in the image scene.
EP 0 935 880 B1 discloses a circuit arrangement by means of which this pulling effect can be suppressed. According to this reference, the image cell known from EP 0 632 930 B1 is supplemented by a further MOS transistor, which is arranged in parallel with the MOS transistor already mentioned before. A sort of reset voltage pulse can be applied via the further MOS transistor, and this can be used for quickly reducing excess charges in the region of the light-sensitive element. These excess charges are the consequence of the fast changing between bright and dark in the image scene.